Mechanisms exist for evaluating communications network availability. These mechanisms are based on detecting and evaluating paths. Data in transit in a communications network is routed via paths, which paths are like routes leading from a source to a destination.
A path to be taken within a network to route data from the source to the destination may be learned in different ways, including:                manually, if each communications network equipment has a static input indicating for each destination the direction in which said equipment must relay received data; this method is not in very widespread use and is often unsuitable for large IP networks; and        automatically, if the communications network equipment use one or more routing protocols: an equipment communicates to its neighbors its local topological knowledge of the network and the addresses that it knows how to reach; it receives in a symmetrical way the same type of information from its neighbors; metrics, i.e. weights, are defined on each link of a communications network; these metrics are the basis for constructing all paths and they enable the equipment to route or transfer a data packet to a destination using the path of lowest weight (this method is called the shortest path first (SPF) method); data of this type in transit between the equipment is called routing data.        
In order to respond quickly in the event of path failure or unavailability, and thus improve availability, modern communications networks therefore learn paths dynamically using routing protocols: as soon as an event occurs in a network, all the network equipment are informed and can thus take the individual decisions necessary to reroute the data via a valid alternative path.
This change is not immediate, however, in particular because inertia affects the propagation of information between the communications network equipment, and moreover it is not certain that there will be another path that is available for routing the data.
The most widely used routing protocols are link-state protocols such as the Intermediate System to Intermediate System (IS-IS) protocol and the Open Shortest Path First (OSPF) protocol.
Telecommunications operators use one or more of the following techniques to find out how long the network has not been able to satisfy the data routing requirement correctly:                collecting alarms from network equipment, with the aim of determining the times of unavailability of paths and/or equipment;        using equipment at various places in the network to inject test data for sending to other equipment in order to calculate the time during which no information has been received; and        collecting available traffic counts in each equipment giving the variations in transported traffic quantity on each network path.        
Those techniques have their drawbacks.
Collecting alarms from equipment assumes that those equipment are in a position to send alarms. When an equipment is faulty or down, it is generally not in a position to transmit alarms signaling its own unavailability. The unavailability of an equipment is therefore detected by neighbors of that equipment that are capable of sending at least some of these alarms. Moreover, processing alarms is a task of lower priority than processing for rerouting data. Such a situation induces latency times in sending unavailability information and makes it impossible to determine accurately the time during which the equipment was really unavailable.
Injecting test data is not sufficiently accurate because the test data is not injected continuously, in order to avoid overloading or congesting the network. Thus the real availability of the network is not obtained with the aid of such injection because it may very well be that data is injected at times at which there is total availability and failure of a path or equipment occurs immediately afterwards. The nevertheless real unavailability of the equipment or path is therefore missed. Moreover, that technique requires the use of numerous injection equipment and consumes network resources uselessly.
The use of traffic counts also induces a latency time in sending information and an unacceptable lack of accuracy. It is not entirely impossible to envisage a functional equipment failing to route any traffic. Such a situation may arise if a metric (i.e. a weight) associated with a path is too high compared to a plurality of usable other paths, with the result that the equipment does not route any traffic because the weight of the paths passing through it is too high.
Thus prior art techniques give rise to problems of accuracy when calculating availability. Those accuracy problems can lead to erroneous interpretation of the availability of a communications network and to decisions being made regarding the architecture of the communications network that are not appropriate, given the real problems of the communications network.